This invention relates in general to sewing machines and in particular to a new and useful device for sewing tubular material workpieces in a sewing machine.
From German OS No. 22 16 373, there is known a device including a rotatable dress form for slipping thereon pieces of garment to be worked. The axis of rotation of the dress form extends through the opening of the piece of garment whose circumferential hem is to be worked. The device comprises a fixed base and a movable frame on which a projecting arm carrying a sewing machine is pivotally mounted. As a basic construction, a drive shaft and a surrounding hollow shaft are mounted coaxially of the axis of rotation of the dress form. To bring the dress form and the sewing machine mounted on the pivotal arm into mutual positions for producing a seam corresponding to the circumference of the garment opening, and to keep them in these positions, a plurality of cams is provided which are secured to the drive shaft and the hollow shaft and control the movements of the mobile frame, of the arm with the sewing machine, and of the dress form which is driven at a selected constant circumferential speed for the hem to be sewed. Two of the control cams are conformable to the circumferential shape of the garment hem and they determine the extension of the seam.
The drawbacks of this prior art design are the necessity for expensive control means and, particularly, the fact that only a definite range of garment sizes can be worked with one set of the control cams, while the device must be re-equipped in a time consuming manner with other control cams for working other sizes of garment. It is further unsatisfactory that with the use of a set of control cams for several sizes of garment, stitches of different lengths are produced in each size because for a single revolution of the dress form, the lengths of the seams are different, depending on the size of the garment. Further, since the circumferential length of the hem to be worked also depends on the cut of the garment, the device requires a still more frequent re-equipment. Anyway, the dress form must be changed frequently. Finally, in this design, the necessity of keeping in stock numerous control cams is expensive in itself.